Activated carbon is useful in the removal of chemical agents such as volatile organic compounds from fluid streams and is also useful as a catalyst substrate for special applications. To remove chemical agents from a fluid stream with activated carbon, the fluid stream is directed adjacent the activated carbon. The activated carbon can be in the form of particles in a packed column, a coating on a substrate, a monolith with passages for fluid flow therethrough, and the like.
It is desirable in some activated carbon applications to have a high rate of fluid flow adjacent to the activated carbon and a low level of back pressure. Thus, packed columns of activated carbon are sometimes unsuitable because of the high level of back pressure created. Formed bodies containing activated carbon and having open passages therethrough, such as a honeycomb-shaped activated carbon monolith, are desirable for applications wherein a reasonably high rate of fluid flow and a low level of back pressure are required, but formation of such shapes with a level of strength sufficient to withstand handling and use as an adsorbent filter is problematic. Activated carbon monoliths formed without a binder do not have sufficient strength for some applications.
U.S. Pat. No. 4,518,704 to Okabayashi et al. discloses a formed body comprising activated carbon and a ceramic material. This structure has improved strength properties but Okabayashi teaches firing at a temperature of 1100.degree. C. for a period from 1 to 4 hours to achieve desired bonding and strength. Firing at such a high temperature and for such a long period of time is economically undesirable.
Another problem with making adsorptive monoliths comprising activated carbon and a ceramic material is that it is difficult to extrude a mixture of activated carbon and ceramic forming material without a high level of water in the mixture due to the high porosity of the activated carbon. To successfully extrude a mixture of activated carbon and ceramic forming material into a shape such as a honeycomb, a water content of 30 to 65 percent by weight is required. This moisture must be substantially removed from the extruded monolith before firing to protect the integrity of the formed monolith. A ceramic article subjected to increased temperature during firing, without first having been relieved of most of its moisture content, will usually suffer significant damage in the form of cracks, pop-outs or explosions due to rapid conversions of its remaining moisture to steam.
Drying of a wet, extruded monolith of ceramic forming material and activated carbon is a sensitive process. An unfired ceramic product generally shrinks as it loses moisture, and a monolith can crack if the rate of moisture loss from the monolith during drying is not uniform throughout the monolith.
Accordingly, there is a need for a formed body comprising activated carbon that can be formed by extrusion, can be dried and fired without cracking, can be fired at more economical conditions such as a lower temperature and a shorter time, has sufficient strength to withstand handling and use as an adsorptive filter, and has a shape which accommodates sufficient fluid flow throughput.